When my oldest was about 18mo we were at a mall play area (like you describe) and something similar happened. An older kid was being pretty rough around ALL the little ones.
and (1) there's an age limit on those small playscapes. and it clearly said no one over the age of 4. maybe he was just tall for his age, dunno. and (2) his mom wasn't watching him too well but she was sitting next to us, chatting with another woman. while (3) her son climbed on TOP of the playscapes standing up and jumping down to the ground almost stomping on little crawlers below; nearly hit our daughter a couple times.
we warned him twice to watch for the small kids.
his mom heard us warn him, did nothing.
then our daughter began crawling in a tunnel.
so he decided it'd be fun to crawl the opposite direction and push her out. the 1st time my husband got up & told the boy to find somewhere else to go bc she is too little to play that rough. the boy did it again so my husband approached the mom (who was watching this all go down!!! and doing nothing!!) and said "are you going to get your son off my baby, or would you like me to, and by the way I'm mad" and she just SAT THERE with her coffee and said "Alex, get off her" (lol!!! what?)
at that point our daughter screamed so my husband pulled the kid by his pant ankles and pulled him out of the tunnel and said "you're done"
then he notified security and they were kicked out.
I DON'T GET SOME PARENTS!
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Yes, you DO say something to other kids (it takes a village) and if the parent has a problem, so what? It's their job to correct them or supervise and if they are not doing it, then SOMEONE HAS TO.... like, say, an adult?